pgsql: Improve stability of identity.sql regression test.

Improve stability of identity.sql regression test.

I noticed while trying to run the regression tests under a low
geqo_threshold that one query on information_schema.columns had
unstable (as in, variable from one run to the next) output order.
This is pretty unsurprising given the complexity of the underlying
plan. Interestingly, of this test's three nigh-identical queries on
information_schema.columns, the other two already had ORDER BY clauses
guaranteeing stable output. Let's make this one look the same.

Back-patch to v10 where this test was added. We've not heard field
reports of the test failing, but this experience shows that it can
happen when testing under even slightly unusual conditions.

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/db7c7b88d00b615f203a4984ef0d01c5e2e46a33

Modified Files
--------------
src/test/regress/expected/identity.out | 2 +-
src/test/regress/sql/identity.sql | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)